I'll chime in here, since I'm building one. Twitter does one thing that I opted to change--everything is public. The short-form, low-effort, public posting that defines twitter leads to a platform that promotes trolling over discourse. Haven[1] instead promotes private discourse where every post is private.

It's fully decentralized in that everyone runs their own instance, but with all open protocols (RSS), it would be trivial for someone to build a centralized/federated instance that can still interoperate. The open protocol which enables a decentralized model is the most important part of that decision.

With RSS as the protocol, you get interoperability with every blog ever for free. Combine private posting with public content and you get something really neat--the ability to have private conversations about public content all on the platform.

You also get an elegant solution to free-speech vs civility. No public content means no discovery so you'll never see anything from a source you didn't explicitly opt-in to. No trolls and no spam.

Arguably, this is more of a Facebook alternative than a Twitter alternative but I still think it is a very valuable approach to creating more civility in our broader society.

[1]: https://havenweb.org